1. Technical Field
The present disclosure relates to driver assistance systems and particularly to a method for detection along a road traveled by a vehicle, the presence of a guard rail or a vertical lane divider using a camera and, more particularly to estimate the lateral distance to the guard rail or the concrete lane divider.
2. Description of Related Art
During the last few years camera based driver assistance systems (DAS) have been entering the market; including lane departure warning (LDW), automatic high-beam control (AHC), traffic sign recognition (TSR) and forward collision warning (FCW).
Lane departure warning (LDW) systems are designed to give a warning in the case of unintentional lane departure. The warning is given when the vehicle crosses or is about to cross the lane marker. Driver intention is determined based on use of turn signals, change in steering wheel angle, vehicle speed and brake activation. There are various LDW systems available. One algorithm for lane departure warning (LDW) used by the Applicant/assignee (Mobileye Technologies Ltd., Nicosia, Cyprus, hereinafter “Mobileye”) of the present application is predictive in that it computes time-to-lane crossing (TLC) based on change in wheel-to-lane distance and warns when the time-to-lane crossing (TLC) is below a certain threshold. Other algorithms give a warning if the wheel is inside a certain zone around the lane marker. In either case, essential to the lane departure warning system is the lane marker detection algorithm.
Typically, the lane markers are detected in the camera image and then, given the known camera geometry and camera location relative to the vehicle, the position of the vehicle relative to the lane is computed. The lane markers detected in the camera image are then collected over time, for instance using a Kalman filter. Wheel-to-lane marker distance may be given with an accuracy of better than 5 centimeters. With a forward looking camera, wheel-to-lane marker distance is not observed directly but is extrapolated from the forward view of the camera. The closer road markings are observed, less extrapolation is required for determining wheel-to-lane marker distance and more accurate estimates of wheel-to-lane marker distance are achieved especially on curves of the road. Due to the car hood and the location of the camera, the road is seldom visible closer than six meters in front of the wheels of the car. In some cars with longer hoods, minimal distance to visible road in front of the car is even greater. Typically the lane departure warning system of Mobileye works on sharp curves (with radius down to 125 m). With a horizontal field of view (FOV) of 39 degrees of the camera, the inner lane markers are still visible on curves with a radius down to 125 meters. In order to correctly perform lane assignment on curves, lane markings are detected at 50 meters and beyond. With a horizontal field of view (FOV) of 39 degrees for the camera, a lane mark of width 0.1 meters at 50 m distance corresponds in the image plane to just under two pixels wide and can be detected accurately. The expectation from the lane departure warning systems is greater than 99% availability when lane markings are visible. Expectation with 99% availability is particularly challenging to achieve in low light conditions when the lane markings are not freshly painted (have low contrast with the road) and the only light source is the car halogen headlights. In low light conditions, the lane markings are only visible using the higher sensitivity of the clear pixels (i.e. using a monochrome sensor or a red/clear sensor). With the more powerful xenon high intensity discharge (HID) headlights it is possible to use a standard red green blue (RGB) sensor in most low light conditions.
The core technology behind forward collision warning (FCW) systems and headway distance monitoring is vehicle detection. Assume that reliable detection of vehicles in a single image a typical forward collision warning (FCW) system requires that a vehicle image be 13 pixels wide, then for a car of width 1.6 m, a typical camera (640×480 resolution and 40 deg FOV) gives initial detection at 115 m and multi-frame approval at 100 m. A narrower horizontal field of view (FOV) for the camera gives a greater detection range however; the narrower horizontal field of view (FOV) will reduce the ability to detect passing and cutting-in vehicles. A horizontal field of view (FOV) of around 40 degrees was found by Mobileye to be almost optimal (in road tests conducted with a camera) given the image sensor resolution and dimensions. A key component of a typical forward collision warning (FCW) algorithm is the estimation of distance from a single camera and the estimation of scale change from the time-to-contact/collision (TTC) as disclosed for example in U.S. Pat. No. 7,113,867.
A recent U.S. Pat. No. 7,411,486 states (column 1, lines 35-37) that lane-departure warning systems which are equipped with only one image-transmitting sensor are not capable of differentiating between edge-of-lane markings and a structural boundary at the edge of the lane. (emphasis added) Consequently, U.S. Pat. No. 7,411,486 discloses a driver assistance system for warning a driver of a motor vehicle of a risk of departure from the lane. The disclosed system includes a camera for detecting edge-of-lane and/or lane markings in the area sensed by the camera, and in addition a distance sensor with which the distance from objects elevated with respect to the surface of the lane can be determined in the region of the edge of the lane, in particular of a structural boundary of the edge of the lane.
Thus there is a need for and it would be advantageous to have a driver assistance system and corresponding method adapted to perform and vertical structural barrier or guardrail detection along the edge of the road or a lane using a camera and without requiring use of an additional sensor for instance to detect distance to the guardrail or barrier.